Fortifications on Mud Island, site of Fort Mifflin, predate the Revolutionary War. The island is in the Delaware River, seven miles below Philadelphia. The British started building the first works, known locally as Mud Fort, in 1771. A British bombardment destroyed the first fort, occupied by Patriot forces, in 1777. In 1795 a new fort erected on the site was named in honor of Pennsylvania's first governor, Maj. Gen. Thomas Mifflin of the Continental Army. In 1798 work began on a new masonry structure to replace the older works, and the new fort was completed two years later. The post was abandoned and reoccupied several times during the nineteenth century. Fort Mifflin was garrisoned during the Civil War, and Eastman was in command when the war ended. As the Army reduced its force structure at the end of the war, the post's garrison grew considerably smaller. The 1875 Surgeon General's report on hygiene noted that although the post had quarters for "one company of artillery," the facility was "in charge of an ordnance-sergeant."
Fort Mifflin became a National Historic Landmark in 1915. During World War I the post was used to store munitions, and its final contribution to national defense was to host an antiaircraft battery during World War II.